With the improvement of a bit rate of communication, a usage method to communication or the like in a higher frequency band (particularly, a terahertz wave domain) has been developed. In a frequency band more than a millimeter wave band, since output of a semiconductor device is lowered, a traveling wave tube, which is an amplification device enabling large output, is used.
A slow wave circuit is one of important components of the traveling wave tube. As the slow wave circuit of the traveling wave tube, a helix type slow wave circuit is mainly used. The helix type slow wave circuit allows an electron beam to pass through an interior of a helix type waveguide and causes interaction between a high frequency signal propagating through the waveguide and the electron beam, thereby amplifying the high frequency signal. That is, the helix type slow wave circuit includes an electron gun that generates the electron beam, a slow wave circuit that allows the electron beam and the high frequency signal to interact with each other, and a collector that captures the electron beam after the interaction is ended (a general description of the traveling wave tube, for example, is provided in Non-Patent Literature 1 (NPL1)).
When a frequency of a signal inputted to the traveling wave tube becomes high and approaches a terahertz wave band, since its wavelength becomes short, micro-fabrication of the slow wave circuit is required. However, in the helix type slow wave circuit, components having a three-dimensional structure are assembled in a structure called an integrated pole piece (IPP). The helix is supported and fixed by a support rod of a dielectric and a permanent magnet is further provided, so that a periodic magnetic field device is formed. It is difficult to high accurately assemble the helix, which has come to be micro-fabricated with a high frequency, by using a complicated structure such as the IPP.
Thus, in the terahertz wave band, a folded waveguide type slow wave circuit is used. This is because the folded waveguide type slow wave circuit is suitable to be manufactured by a micro electro mechanical systems (MEMS) manufacturing technology or a lithography technology. The folded waveguide type slow wave circuit is achieved by a combination of a folded waveguide, through which a high frequency passes, and a beam hole through which an electron beam passes.
The sectional shape of the beam hole of the folded waveguide type slow wave circuit is ideally a circle. The circular beam hole can be easily manufactured in precise machining in the folded waveguide type slow wave circuit used in a low frequency band. Normally, a slow wave circuit is divided and is subjected to machining and assembling, so that a folded waveguide type slow wave circuit is completed (NPL1).
As a frequency increases from a microwave to a terahertz wave, a wavelength is shortened. Accordingly, micro-fabrication of a waveguide is required. However, it is difficult to employ a machining technology as a manufacturing technology for micro-fabrication of a folded waveguide. In this regard, manufacturing using a lithography technology or the like is performed (Patent Literature 1 (PTL1)).
As a representative fine processing technology used for manufacturing the folded waveguide, there is a lithographie galvanoformung abformung (LIGA) technology using UV light or X ray (synchrotron light) used in MEMS manufacturing.
In the case of forming a circular section beam hole by using such a fine processing technology, since the number of manufacturing masks increases in order to reliably reproduce a curve and a manufacturing process is complicated, there is a disadvantage of yield deterioration or the like. Therefore, in a background art, the folded waveguide type slow wave circuit is manufactured in which the sectional shape of the beam hole is designed as a quadrilateral (Non-Patent Literature 2 (NPL2)).